It will appreciated by one having ordinary skill in the art that trees and other woody plants are not the only source of fibers for papermaking. There are a variety of nonwood annual and perennial plants which produce fibers having sufficient strength and length to produce paper with acceptable qualities. These nonwood plants are often referred to in the art as "agricultural residues" or "fiber crops". Examples of plants for each of these categories include:
Agricultural Residues Fiber Crops Wheat straw Kenaf Rice straw Industrial hemp Corn stalks Sisal Bagasse (sugar cane) Textile flax straw Rye grass straw Hesperaloe Seed flax straw Flax straw
One of the main advantages of these fiber sources is that they are perceived in the art as environmentally-benign alternatives to the use of trees. Indeed, nonwoods are currently the major source of papermaking fiber for some developing countries and countries lacking significant wood resources.
For the most part, however, the development of a nonwood fiber industry in North America has been retarded due to the fact that nonwood pulps are usually more expensive on a per-ton basis than wood pulps. Recently, several factors have dramatically increased the level of industry interest in these nonwood fiber sources. Some of these factors include environmental pressure to stop using trees; projections of world fiber shortage by 2010 and the need to find alternative fiber sources; abundance of agricultural residues (such as corn stover and wheat straw) that are otherwise burned off fields; and opportunities to produce multiple products (oils, textile fibers, papermaking fibers, board fibers, plastics, food) from a simple fiber source, which provides unique opportunities for sustainable agriculture.
However, effective use of nonwood fiber sources presents some significant challenges that must be overcome. These challenges include the following:
(1) nonwoods must be harvested annually and stored, and thus, are sensitive to growing season, harvest conditions, etc.; PA1 (2) nonwoods have a low bulk density compared to trees, and thus, can be hard to store and transport; PA1 (3) nonwoods may require larger amounts of herbicides and pesticides as compared to trees; PA1 (4) nonwoods generally require smaller pulp and paper mills due to transport constraints, and it is often difficult to establish efficient chemical recovery systems for small mills; and PA1 (5) many, but not all, nonwoods comprise fibers that may be shorter, more slender, or weaker than wood fibers. PA1 (1) The process should have a minimal number of processing steps, or stages; PA1 (2) The process should utilize a minimal amount of equipment; PA1 (3) The equipment should be as simple and low-cost as possible; PA1 (4) The process should minimize water usage by: PA1 (5) The process should use readily-available chemicals at reasonable dosage levels; PA1 (6) The process should be odor-free and optionally, chlorine-free; and PA1 (7) The process should use chemicals which permit recovery of all internal filtrate streams. PA1 (1) The final pulp should have a brightness in the 70-90% ISO range for paper grades made in an integrated pulp and paper mill, and 85-90% ISO for high-end and market pulp grades; PA1 (2) The pulp should have adequate strength properties, i.e. the fibers should be subjected to minimum damage; PA1 (3) The drainage rate (freeness) of the pulp should be sufficiently high so that the pulp can be formed and dewatered on a typical paper machine; and PA1 (4) The process should be able to remove the high content of pith, parenchymal cells, fines, and other non-fibrous materials often found in nonwoods; these materials make the pulp "dirty" and also cause slow drainage.
Agricultural residues represent an economically-promising source of nonwood fibers. The low bulk density and high transport costs of agricultural residues suggests a nonwood mill capable of producing 50-350 tons of pulp per day. This "mini-mill" must produce pulp which can compete with wood pulp produced in very efficient "mega-mills" producing 1000-3000 tons per day. To make the situation even more challenging, it is generally not possible to simply scale down the wood pulp processes, which rely on large production volumes to justify the high capital costs of equipment.
In order to be successful, nonwood mini-mills must therefore make use of processes which are cost effective and environmentally sound at small scale. Such processes should ideally meet the following criteria:
(a) recycling as many filtrate streams internally as possible, PA2 (b) minimizing the number of dilution and thickening stages required, PA2 (c) minimizing the number of washing stages required, and PA2 (d) minimizing the number of pH changes required;
Given the fragile nature of agricultural residues and the quality requirements of the printing and writing grade paper markets, the successful mini-mill process should also meet the following criteria:
Thus, a substantial challenge in reducing nonwood raw materials into fibers for papermaking is to find a pulping method for application in a mini-mill setting which addresses the criteria set forth above. The term "pulping" is generally defined as the reduction of the bulk fiber source material into its component fibers. The key is to perform this reduction without damaging the fiber (thereby reducing strength) or without losing too much fiber that will be suitable for papermaking (termed a "yield loss").
Several classes of pulping processes are generally known in the art. These processes include the following:
(1) Chemical Pulping--In this type of pulping, a large chemical dose is used to dissolve away most of the lignin (glue) which holds the fibers together in the raw material. This dissolution is carried out in a digester, where chemicals are mixed with the raw material and then heated to medium to high temperature (100-170.degree. C.) and high pressure (2-15 atm). Standard digestion processes are carried out for about 1-8 hours. At the end of the digestion, the fibers are washed to separate them from the liquor, which contains dissolved lignin and spent chemicals. Elaborate systems have been developed to thicken and burn the liquor in order to recover heat energy from the lignin and regenerate the chemicals for use in subsequent digestion procedures.
Pulps from full chemical processes are characterized by high purity (high cellulose content, low hemicellulose and lignin content), suitable cleanliness levels, and suitable strength. With subsequent bleaching, high-brightness pulps for demanding printing and writing grade paper products may be produced. However, the processes often have a low yield (30-50%) due to chemical dissolution. In addition, full chemical processes require high capital investment and high operating costs. Thus, standard full chemical pulping processes are generally not suitable for nonwoods pulping applications in mini-mills.
(2) Mechanical Pulping--In this type of pulping, raw materials are separated into fibers using brute mechanical force. Usually, the raw material is placed between rotating refiner plates, which shear it apart. Heat can be applied to soften the fibers prior to refining. Yield from these types of processes is typically high (65-95%), but the quality of pulps is usually inferior to chemical pulps. Because there is still a large amount of lignin on the fiber surfaces, bonding sites are blocked, resulting in lower strength properties. Sheet flexibility is also reduced because lignin is left in the fiber walls. Overall, mechanical pulps are useful only for low-end paper grades like newsprint or catalog. However, since large quantities of chemicals are not required, chemical recovery is no problem. In addition, capital and operating costs are manageable. However, because of the limitation on pulp, and subsequently paper, quality described above, a purely mechanical process is also believed to have limited application to the pulping of nonwoods due to the fragile nature of many nonwoods.
(3) Chemi-Mechanical Pulping--This type of pulping uses aspects of both of the previously described process types. Raw material is impregnated with small amounts of chemicals to soften the lignin, and then it is mechanically treated to complete the separation. Heat is typically applied to improve pulping. With this hybrid process, good fiber properties may be developed without extensive chemical application. In addition, capital and operating costs are almost as low as for pure mechanical pulping. Pulps from chemi-mechanical processes can be used for low- to medium-quality papers, and with additional processing they may be used for some high-end purposes. However, a chemi-mechanical pulping process suitable for the pulping of nonwoods has not been described in the art.
Background art patents include U.S. Pat. Nos. 4,756,799 and 4,900,399, both to Bengtsson et al., describing methods for manufacturing bleached chemi-mechanical and semi-mechanical fiber pulp by means of a one or two stage impregnation process. However, these patents particularly describe production of a pulp from wood materials. The described methods particularly require that the wood material be preheated before it is subjected to mechanical manipulation in a standard twin disk refiner. Thus, the method steps described in these U.S. patents are not believed to have particular application to a pulping process for nonwood m materials.
U.S. Pat. Nos. 4,997,488 to Gould et al.; 4,806,475 to Gould; U.S. Pat. No. 4,774,098 to Gould et al.; and, 4,649,113 to Gould describe alkaline peroxide treatment of nonwoody lignocellulosics and products made by such treatments. However, the primary focus of these patents is the production of nutritional supplements, culture media or other compounds from cellulose in the nonwoody materials for use in feeding domestic animals, humans, or in the growth of microbial cultures. Stated differently, the focus of the methods described in these patents is to produce materials from cellulose that can be metabolized by animals.
Despite the disclosure of the foregoing U.S. patents, there has been no suggestion in the art of a pulping process for nonwood fiber source materials that meets the criteria set forth above. Indeed, for nonwoods the most sensible approach is to install a small mill at the center of a defined growing area. As noted above, this mill should use a simple process, with low operating and capital costs, to maintain economies of scale equal to those of the mega-mill. The process should render the mill almost invisible to the environment, and should require only small amounts of environmentally-benign chemical agents. Such a process is not currently available in the art.